Recently, the securing of new sources of energy has garnered worldwide attention due to the depletion of fossil fuel sources and global warming. Accordingly, the importance of energy storage for the efficient use of energy, together with the development of new and renewable sources of energy is on the rise.
Particularly, in the automobile field, the development of electric vehicles has been urgently demanded, due to the depletion of fossil fuels. However, when common lithium secondary batteries are used as energy storage devices in electric vehicles, trips taken at distances over about 200 kilometers are somewhat difficult to undertake on a single charge. In addition, the common lithium secondary battery is not appropriate for storing electrical power generated by new and renewable energy sources over long periods of time.
In this regard, the development of a novel material having high capacity and high output and design techniques is necessary. Particularly, attention is focused on the development of anode materials, as the development of cathode materials has stagnated due to inherent limitations of materials commonly used therefor. As the anode material of the secondary battery, graphite-based materials are used, however, such graphite-based materials have a low capacity (theoretical capacity: about 372 mAh/g, about 830 mAh/ml; reversible capacity: about 330 mAh/g). Thus, the development of a replaceable anode material having a capacity of about 500 mAh/g or more is emerging as a new material for use in the realization of a high capacity lithium secondary battery.
Si-based materials have been prominent as replaceable anode materials instead of graphite-based materials. The greatest advantage of such Si-based materials is a storage capacity of about 4 times by unit volume, and about 10 times by the unit weight when compared with that of graphite. In addition, when a battery is manufactured with LiCoO2, LiMn2O4, or the like, about 3.4 V may be obtained, a voltage level lower by only 0.3 V than a presently used battery having a voltage of 3.7 V. Thus, when an anode material, stable during charging and discharging, is formed, a battery containing such a material may be immediately commercialized.
When considering the weight and volume thereof after a reaction with lithium was conducted, graphite-based materials exhibited a small change; however, metal-based materials, including Si-based materials, exhibited a very large change in volume. In the case that lithium is intercalated into graphite and an intercalation reaction occurs therebetween, lithium is present between graphite layers, and the structure of the graphite remains largely intact. However, metal elements such as Si have an alloying reaction when combined with lithium, and an alloy phase (LixM) having a new structure and composition may be formed. The alloy phase formed by the reaction of the metal element with lithium has an ionic phase, and a high degree of brittleness may be present. Thus, when a volume of such a material increases, mechanical stress may be generated, and mechanical breakage may easily occur. Therefore, when a metal-based material including Si is used as an anode material of a lithium secondary battery, cracks may be generated in an electrode, and electrical contact between an active material and an electrode may be deteriorated. In this case, capacity may largely decrease according to the progress of a cycle, and a battery lifespan may be decreased.
Meanwhile, a solid phase reaction method may be used as a method for manufacturing an anode material. A typical solid phase reaction method is a gas phase spraying method. A gas phase spraying method is a method for preparing silicon oxide in which an amount of metallic silicon is increased. In such a gas phase spraying method, a particle diameter of silicon oxide may be relatively easily controlled by controlling the conditions of a preparation reaction, and thus, particle size distribution may be good, and a clean surface may be obtained. However, in the case of a gas phase spraying method, the selection of particle size of minute particles may be difficult, repeated sintering and pulverizing processes may be necessary, and manufacturing costs and manufacturing times may be greatly increased. In addition, particle size uniformity and the homogeneity of chemical compositions may be problematic.
To solve the above-described issues, a liquid phase preparation method has been developed, of which a sol-gel method is typical. When a transition-metal oxide powder is prepared using the sol-gel method, nanoscale lithium ions and nanoscale transition-metal ions are mixed. Thus, an active material having a very small particle size, a large surface area, homogeneous particle size distribution, and a homogeneous composition may be obtained through a sol-gel liquid phase preparation method as compared to a powder prepared by a solid phase reaction method.
A sol-gel reaction basically includes a hydrolysis reaction and a condensation reaction. A colloid is a suspension in which the size of dispersed-phase particles distributed therein is microscopic, gravitational force affecting particles is negligible, and interactions between particles are controlled by Van der Waals force or an influence such as a surface charge, or the like. This kind of suspension may produce a polymer that may form particles, and a precursor for forming the suspension includes a metal-alkoxide such as methoxide, ethoxide, propoxide, butoxide, or the like, a metal-acetylacetonate, a metal-acetate, etc.
When such a precursor reacts with water, a hydrolysis reaction may proceed at rapid rate, and this reaction may be completely completed or partially completed according to the amount of water or the influence of a catalyst. Once the hydrolysis reaction is completed, even partially, the condensation reaction may proceed while generating water or alcohol, and finally, a polymer in which all molecules are connected may be formed.
An independent solid in which all molecules present in a sol make bonds through a liquid medium, and fluidity is eliminated, is referred to as a gel. To form the gel as described above, a hydrolysis reaction for forming an OH-bond is necessary, and this reaction is dependent on (or sensitive to) the pH of a solution.
Basically, a condensation reaction occurs simultaneously with olation and oxalation reactions. An olation reaction is a reaction in which the formation of a hydroxyl bridge occurs, allowing for a condensation reaction with partially condensed additional units to continuously occur, while an oxalation reaction is a reaction in which the forming of an oxo bridge between the central atoms of metal molecules occurs.
In addition, the rates of the hydrolysis reaction and the condensation reaction may vary, according to the properties of a metal ion M. When such reaction rates are different, and the hydrolysis reaction of an optional metal M occurs continuously, the condensation reaction of an additional metal M″ may be conducted. In such circumstances, the overall chemical composition of a gel may become non-uniform. Thus, a chelating agent forming a bidentate with a metal ion to control the reaction rate may be added to control the rates of hydrolysis and condensation, to ultimately obtain a gel having uniform distribution of a chemical composition. The chelating agent may include various compounds such as PAA, PVB, citric acid, glycolic acid, glycine, ethylene glycol, and the like, and it has been reported that a gel having good properties may be obtained by using the chelating agent.
According to the above-described methods, the manufacturing of a commercialized powder has limitations in terms of using a relatively expensive starting material, reaction products during an oxidation, a reduction, a preparation reaction, and restrictions on the size of particles.
To solve the above-described defects, a method of preparing silicon oxide is provided in Patent Documents 1 to 3, including inserting a mixed raw powder including a silicon dioxide powder into an inert atmosphere under a reduced pressure, heating at a temperature of 1,100° C. to 1,600° C., generating SiO gas, continuously or periodically supplying oxygen to the SiO gas, and precipitating a mixture of gases on the surface of a cooled gas. However, according to this method, a process for preparing a SiO gas is necessary, and a decompression process, as well as the cooling of the mixture of gases, is necessary. Thus, process control is not easy, and an amount of a final product prepared by the preparation process may be somewhat low. Thus, the commercialization of such a powder is somewhat problematic.
In addition, Patent Document 4 provides a method including mixing a silicon dioxide powder and a silicon powder having a hydrogen concentration of 30 ppm or more, heating the mixture to a temperature of 1,250° C. to 1,350° C., vaporizing SiO, precipitating the SiO from a precipitation gas, and pulverizing the SiO. However, according to this method, the limitation of a process is accompanied by the control of a reaction gas, and high costs may be incurred due to an increase in electricity usage.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2002-260651    Patent Document 2: Japanese Laid-Open Patent Publication No. 2011-142021    Patent Document 3: Japanese Laid-Open Patent Publication No. 2001-243535    Patent Document 4: Korean Laid-Open Patent Publication No. 2007-0020130